CUDA device index is not forwarded to the grad tensor

Context

I'm working on allowing CUDA based training in AIDGE on any device, but I'm facing several difficulties with the device index handling.

Problem

When setting a tensor backend, we must ensure that the backend of its gradient tensor is properly set too, and this actually the case.

But the same should apply to the device index, and in practice this is not the case : the device index stays at zero !

Reproduce

To reproduce the problem one could use the following block of code ...

Note : to execute this code one should pybind the device() method

import numpy as np
import aidge_core
import aidge_backend_cpu
import aidge_backend_cuda

array = np.random.rand(4, 4)

tensor = aidge_core.Tensor(array)
tensor.set_datatype(aidge_core.dtype.float32)

tensor.set_backend('cuda', 3)

print(tensor.backend(), tensor.device())
print(tensor.grad().backend(), tensor.grad().device()) # <- device is still 0 !

returns :

cuda 3
cuda 0

EDIT : Solved by !445 (merged)